Today I noticed something very weird while listening to music via headphones on my PC. Everything was fine up to the point where I connected my iPhone 5 for charging. Then suddenly, I had massive bus noise and the fun thing was that the noise was not influenced by the PC, but the activity on the iPhone. It was much louder when I was moving my finger over the touchscreen for example or the iPhone was animating stuff such as opening a folder on the home screen. Some noise was also present when the screen was off.

Is this something caused by the iPhone itself not behaving correctly, or is it the sound card's / PC's fault?

Edit: BTW, just to be clear, I am not talking about that typical noise that occurs when the iPhone is sending data for example.

It happens with every USB port, but with the front ones being louder than when connected to the rear ones. The funny thing is that I have no problem when connecting HDDs, card readers, whatsoever. Only happens with the iPhone.

Assuming you're using a front panel audio jack on your case for the headphones, try plugging the headphones into the rear audio jack and see if it still happens. Sometimes those can be really finicky when it comes to interference issues. Perhaps the iPhone's DC input isn't filtered and the rapidly fluctuating current draw is inducing noise on the +5V power line in the computer. You might try a powered hub to see if that changes anything.